An increasing number of hardware attachments are available to expand upon the functional capabilities of mobile computing devices, such as smart phones. In some instances, card reader attachments, which enable the reading of payment information from a magnetic strip (such as a credit card), can be coupled to a host device via a standard 3.5 mm headphone port. However, due to limitations in transmitting data over such audio ports, some conventional card readers do not possess bi-directional communication capabilities. One impediment to bidirectional reader communication is the wide variation in configurations between different types of host devices. For example, different smart phones or tablet computers can possess a large degree of variation in the way they send and receive audio signals.